The caps are usually attached to the neck of the bottle by means of a screw thread. They can be provided with a safety strip which is broken when the bottle is opened. Such bottle caps produce a large amount of waste material which is difficult to reprocess. The plastic coating is quite firmly embedded in the cap and can be removed only with a great deal of effort. On the other hand, the plastic coating hinders the reprocessing of the aluminiun. The plastic residues would result in such contamination of the smelted aluminium as to make it unsuitable as a raw material.
The plastic coating in the bottle cap usually consists of PVC. This material, in contrast to many other plastics, does not affect the taste of the drink and can therefore not readily be replaced by some other substance which presents fewer problems during reprocessing.
On the other hand, the reprocessing of such materials, in particular bottle caps, has major advantages. First of all, the reprocessed aluminium constitutes an inexpensive raw material, in any case in comparison with production from bauxite. In the case of bottle caps, a closed cycle would be achievable, in which the aluminium can be continuously used for new bottle caps.
Account must also be taken of the fact that the fabrication of bottle caps makes use, as the starting material, of a plate or roll of aluminium which is printed beforehand with the desired lettering, designs and the like. From these plates the caps are then punched, waste material remaining behind which is partially printed. This material remaining behind when the caps are punched out would likewise have to be reprocessable at the same time, in order for a complete cycle to be achievable.